1. Field of the Invention
The present invention relates to a blade containment system for a gas turbine engine, and, more particularly, to a blade containment system including an annular casing having a plurality of ribs formed thereon in order to limit the propagation of cracks and holes formed in the annular casing.
2. Description of Related Art
Conventional high bypass ratio turbofan engines, which are included in the more general category of gas turbine engines and which may be used for aircraft propulsion, typically include a fan, booster, high pressure compressor, combustor, high pressure turbine, and low pressure turbine in serial axial flow relationship about a longitudinal centerline axis of the engine. The high pressure turbine is drivingly connected to the high pressure compressor via a first rotor shaft, and the low pressure turbine is drivingly connected to both the fan and booster via a second rotor shaft, The fan includes an annular disk and a plurality of radially extending blades mounted on the disk, wherein the disk and the blades are rotatable about the longitudinal centerline of the engine. The engine further includes an annular fan casing which surrounds the blades in radially spaced relationship.
Gas turbine engines typically operate at relatively high rotational speeds and, for high bypass ratio turbofans, the fan blades typically include a relatively large radially extending length. Consequently, the fan blades possess relatively high kinetic energy even when the blades are fabricated using lightweight alloys or composite materials. Containment of fan blades has been a continuing problem for the industry, as various known events may occur during engine operation, such as blade contact with foreign or domestic objects, which may cause a blade to be released from the fan disk.
In order to prevent such fan blades or fragments thereof from penetrating the annular fan casing, various containment systems have been employed. While such containment systems have included the use of an annular fan casing manufactured from a high strength material with an adequate radially extending shell thickness to absorb the kinetic energy of the impacting fan blade, more recent containment systems have employed various nesting areas defined by inner and outer annular casings which may include honeycomb structures therein. In addition, ballistic material may be wrapped around the fan casing between the honeycomb structure and the outer casing. An example of such a blade containment structure is found in U.S. Pat. No. 4,534,698 to Tomich.
It has been found that once a hole or crack has been formed in the inner casing of such prior art fan containment casings that such cracks propagate throughout the inner casing. Because the inner casing is the primary load carrying structure of the fan containment case, the size of such a crack or hole caused during a fan blade-out event must be minimized. By doing so, the overturning moment imparted on the engine structure is minimized since the fan case acts as a "journal bearing" for the fan during spool-down of the engine after a blade-out event, whereby the rubbing action of the fan on the case reduces the orbiting of the fan rotor and therefore the overturning moment on the engine structure. Moreover, a large hole or crack in the inner casing reduces its ovalization stiffness, and may allow the fan to orbit into the hole if the hole becomes too large. Additionally, an unarrested crack may propagate to a point where the fan cowl and containment case become detached from the engine, which is unacceptable.